1. Field of the Invention
The process and apparatus of the present invention relates generally to bioconversion of biodegradable feedstocks to provide decomposition of organic matter and production of gases such as methane and relates more particularly to anaerobic digestion of particulate containing biodegradable feedstocks in a non-mixed vertical flow reactor wherein solids, including feed and microorganisms are passively concentrated in the upper portion of the digester to provide increased solids and microorganism retention times and reduced hydraulic retention times.
2. Description of the Prior Art
The production of methane and other usable gases by anaerobic digestion of various organic wastes, particularly sewage sludge organic waste, is well known. The organic feed mixture which provides the substrate for anaerobic biodegradation may comprise a wide variety of organic carbon sources, ranging from raw sewage sludge to municipal refuse, or biomass material such as plants and crop wastes. Anaerobic digestion of organic feedstock generally involves hydrolysis fermentation of organic feedstock to acidic intermediates by acid forming bacteria and conversion of the acidic intermediates to useful gases, such as methane, by methane-producing organisms. Many digester designs, feedstock mixtures and additives have been proposed to increase the methane yield from anaerobic digestion and to provide greater conversion efficiency of organic materials to useful products.
Early designs of sewage digesters attempted to biodegrade sewage sludge for the purposes of sludge volume and odor reduction in an unmixed digester, but they were generally unsuccessful because they failed to provide adequate control of solids inventory, and they developed serious problems such as scum buildup, temperature fluctuations, unequal microbial activity and limited contact between the organic material and the bacteria. Most anaerobic digesters installed recently for biological conversion of biomass and community wastes are of the continuously stirred tank reactor design and provide complete mixing of the reactor contents. Solids and hydraulic retention times are equal in continuously stirred tank reactors and both hydrolysis fermentation reactions converting organic materials to acidic intermediates and methane-producing reactions converting acidic intermediates to methane and other gases occur throughout the reactor. These digesters actively discourage the accumulation of floating or settled solids. The history of anaerobic treatment of wastes and digester design, including more recent process developments, is set forth in P. L. McCarty, "One Hundred Years of Anaerobic Treatment", presented at the Second International Conference on Anaerobic Digestion, Travemunde, Germany, Sept. 7, 1981.
Many organic feedstocks have a relatively low suspended solids content, for example less than about 10 percent suspended solids. The high water content of these types of organic feedstocks causes washout of feed solids and microorganisms from continuously stirred tank reactors at high feed loadings due to high dilution rates. Washout of feed solids and microorganisms results in reduced conversion efficiency and unstable digester conditions. Shorter feed solids retention times in the digester, washout of slow growing methanogenic bacteria and accumulation of inhibitory acidic fermentation products contribute to low conversion efficiency and reduced methane production.
An Upflow Anaerobic Sludge Blanket (UASB) process has been developed for bioconversion of feedstocks which contain primarily soluble organic waste wherein small amounts of solids, ordinarily less than 1 percent of the feedstock, and the bacterial mass are allowed to settle in the reactor. The Upflow Anaerobic Sludge Blanket process and reactor are described in the following publications: G. Lettinga, et al, "Anaerobic Treatment of Methanolic Wastes", Water Research, Vol. 33, pp. 725-737, Pergamon Press Ltd. 1979; and G. Lettinga, et al, "Upflow Sludge Blanket Processes", 3rd International Symposium on Anaerobic Digestion, 1983, Cambridge Mass.; and G. Lettinga, et al, "Anaerobic Treatment of Raw Domestic Sewage" at Ambient Temperatures Using a Granular Bed UASB Reactor, Biotechnology and Bioengineering, Vol. XXV, pp 1701-1723, 1983. This reactor design is limited to liquid feedstocks containing less than about 1 percent solids, and it requires effective gas/liquid separators, recycle for bed expansion, and means for distributing the feed over the bottom of the reactor.
Anaerobic filter-type reactors promote the retention of bacteria in the digester by attaching bacteria to fixed inert materials in the digester. Anaerobic filter-type digesters are also limited to primarily liquid feedstocks containing less than about 1 percent solids since they become plugged when solids concentration in the digester increases due to higher solids loading or accumulation of solids over longer periods of operation.
Horizontal plug flow digester designs have been implemented, but horizontal plug flow reactors are limited to use of homogeneous solids feed materials (such as manure), which do not tend to settle by gravity. The horizontal plug flow reactor design encourages rapid disengagement of gas from the liquid phase. Horizontal plug flow reactors generally have poor conversion efficiencies of the biodegradable fraction, on the order of about 40 to 60 percent due to biologically unreactive zones within the digester, shortcircuiting of the feed material, and bacterial washout.
Continuous flow fluidized bed fermenters embodying a tower design or a supported film reactor are described in G. F. Andrews, "Fluidized-Bed Fermenters: A Steady-State Analysis", Biotechnology and Bioengineering, Vol. XXIV, pp 2013-2030, 1982. This article teaches that stratification tends to occur in tower fermenters, and solids concentration varies along the height of the tower fermenter, with a low cell concentration in the upper parts of the tower fermenter leading to a low volumetric productivity.
U.S. Pat. No. 4,208,279 teaches anaerobic digestion of animal waste which is fed to the top and one end of an unstirred digestion volume which is about five times as wide as it is high, and effluent sludge is removed at the opposite end of the reactor. Solids movement in the digester is essentially horizontal and the liquid volume is not agitated, except by gas formation. Suitable solids residence times are one month and over, and the solids feed concentration is about 5 percent.
U.S. Pat. No. 4,311,593 teaches anaerobic digestion of waste water in a digester volume which is about four times as wide as it is high with microorganisms stabilized on high surface area media and providing agitation of the microorganism biomass on the media by gas formation bubbling up through the reactor liquid. U.S. Pat. No. 4,388,186 teaches mechanical condensation of sludge prior to anaerobic digestion of sludge in a vertically elongated, stirred digester tank. The '186 patent also teaches conducting an acid fermentation stage and an acid regression stage separately prior to carrying out an alkaline fermentation stage in the elongated, stirred digester tank. U.S. Pat. No. 1,806,698 teaches a sludge digester wherein solids collect at the bottom, and supernatant liquid accumulating in the upper portion of the digester is recycled to the surface of the digester contents to reduce foam scum. U.S. Pat. No. 1,880,773 also teaches anaerobic digestion of sewage sludge in a digester wherein solids settle to the bottom of the tank and liquid supernatant from the uppe portion of the digester is recirculated to prevent the accumulation of scum or foam at the top surface of the digester contents.
U.S. Pat. No. 4,505,819 teaches top feeding of untreated organic materials in an aqueous medium to a contact reactor, the aqueous bottom contents of which is fed to a fluidized bed anaerobic digester comprising anaerobic bacteria supported on finely divided inert solids. U.S. Pat. No. 4,157,958 teaches anaerobic gasification in a single vessel which is inverted and submerged in a body of water providing both a trap for material in the reactor and also providing liquid agitation to the organic feed material.
Anaerobic digestion of terrestrial plant material to produce methane has been recognized as exemplified by D. L. Klass and S. Ghosh, "Methane Production by Anaerobic Digestion of Bermuda Grass", presented at Symposium on Biomass as a Non-Fossil Fuel Source, ACS/Chem. Soc. of Japan Joint Chemical Congress, Honolulu, Hawaii, Apr. 1-6, 1979. Likewise, the anaerobic digestion of aquatic plant material to produce methane has been recognized as exemplified by R. P. Lecuier and J. H. Marten, "An Economic Assessment of Fuel Gas from Water Hyacinths", Symposium papers, Clean Fuels from Biomass, Sewage, Urban Refuse, Agricultural Wastes, Orlando, Fla., Jan. 27-30, 1976.
U.S. Pat. No. 4,329,428 teaches production of methane gas in higher yields and at higher rates by thermophilic and mesophilic anaerobic digestion of a mixture of plant material of terrestrial or aquatic origin and organic waste. U.S. Pat. No. 4,424,064 teaches production of methane gas with higher yields and at higher rates by thermophilic or mesophilic anaerobic digestion of aquatic plant material, at least a portion or all of which has been grown in organically polluted water. U.S. Pat. No. 4,316,961 teaches higher yields of methane gas at higher rates by thermophilic or mesophilic anaerobic digestion of plant material and/or organic waste of normally low biodegradability in the presence of an extract of different plant material.
Separated two-phase anaerobic digestion processes, wherein in a first acid phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous matter to volatile fatty acids of low molecular weight and in a second methane phase, methanogenic microorganisms convert the volatile fatty acids to product gas composed primarily of methane and carbon dioxide, have been found to enhance conversion efficiency. U.S. Pat. No. 4,022,665 discloses certain specific operating conditions for a two-phase anaerobic digestion process in separated vessels which promotes more efficient conversion of organic material.